This investigation seeks to characterize the interaction of the factor VIII complex with thrombin, in particular the biochemical mechanism by which thrombin activates factor VIII procoagulant activity and subsequently inactivates the activity. An understanding of the regulatory role of this interaction in normal hemostasis will lead to a better u nderstanding of the basic hemostatis process and of the functional defects in the factor VIII deficiency diseased (Hemophilia A and von Willebrand's disease) as well as in pathologic thrombotic states.